What exactly does the graph of cos(x) look like from 0 to 360 degrees?
It starts at cos(0)=1,
drops to zero at 90 degrees,
goes down to -1 at 180 degrees,
comes back up to zero at 270 degrees,
reaches 1 again at 360 degrees.
Therefore there is an interval from zero to where cos(x) is
and similar at the other end of the period.